1. Field of the Invention
The present invention relates to a varnish and a prepreg, in particular to a halogen-free varnish with composite curing agent, a prepreg made out of the varnish, and a substrate made out of the prepreg.
2. Description of Related Art
Printed circuit board (PCB) is widely used in many applications and fields, for example, electronic devices of electronic products are inserted on PCBs. Now, PCBs are increasingly applied to high power and high thermal environment. Therefore, study and research on heat-dissipation are developed in order to improve the heat-dissipation efficiency of PCBs.
PCB is made by pressing the prepregs, copper clad laminate (CCL), or copper film. The prepregs are made by dipping glass fabrics into varnish and then curing the fabrics to form thin prepregs.
Regarding environmental protection, halogen-free PCB is more and more developed for electronic application. Phosphor is usually added into the halogen-free PCB for compensating the anti-flammability of the PCB. Traditionally, the varnish can contain either primary resin with phosphor or curing agent with phosphor. Alternatively, a phosphoric compound may be added into the varnish for improving the anti-flammability of the PCB.
The traditional varnish contain phenolic epoxy resin as the primary resin, phenol resin as the curing agent, and phosphoric compound so as to manufacture a prepreg with anti-flammability. Patent NO. TW 293831 and Patent NO. TW 583258 discloses a composite curing agent mixing the phenol resin with benzoxazine (Bz) ring along with another curing agent of phenolic resin. However, the Bz resin can lower the moisture absorption and has a low dielectric property, but the toughness of the prepreg is low. Moreover, the heatproof property is lowered in the case of the over-addition of the Bz resin. In addition, the prepreg has low reactivity for the lamination process of the FP4 substrate and the glass transition temperature (Tg) of the substrate only reaches 160° C.